Theoretical analysis of magnetoplasmonic interferometers for sensing

Comunicación presentada en la Conferencia Española de Nanofotónica (CEN2012), celebrada en Carmona (Sevilla) del 1 al 4 de octubre de 2012.Peer Reviewe

Plasmonic and magnetoplasmonic interferometry for sensing

[Introduction and background] Nowadays, we are surrounded by sensors in our daily lives, in industrial processes, medical diagnosis systems, environmental monitoring, etc. The development of sensors with higher sensitivity and smaller dimensions to be integrated in miniaturized systems is then of highest importance for our society. Within all different kinds of sensors, optical sensors are advantageous because they are highly versatile, non-invasive and they can be used in aggressive conditions. In particular, sensors based on surface plasmons, known as surface plasmon resonance (SPR) sensors, have become increasingly popular in biosensing in recent decades due to their high sensitivity and ease of use. Different SPR configurations, such as modulation techniques, have been proposed and demonstrated in order to increase their sensitivity, and attempts to obtain miniaturized SPR sensors have been carried out, the development of plasmonic interferometry sensors being a promising path.[Main results] We have compared theoretically the performance of three implementations of plasmonic sensors: the standard SPR configuration, plasmonic interferometry and magnetically modulated plasmonic (magnetoplasmonic) interferometry. Our results show that the sensitivity of plasmonic interferometers surpasses that of standard SPR methods for long enough except the micrometer-sized interferometers. Moreover, when plasmonic interferometers are magnetically modulated, the direct measurement of the induced modulation in the surface plasmon wavevector allows one to further increase the system sensitivity.[Wider implications] These results show that the development of plasmonic interferometers for sensing, either plain or magnetically modulated, is an interesting route to obtain miniaturized surface plasmon based sensors with higher sensitivity. Moreover, the extended knowledge of immobilization protocols in gold already developed for standard SPR sensors will remain applicable.We acknowledge funding from the Spanish MINECO ('MAPS' MAT2011-29194-C02-01 and 'FUNCOAT' CONSOLIDER INGENIO 2010 CSD2008-00023) and the Comunidad de Madrid ('MICROSERES-CM' S2009/TIC-1476).Peer Reviewe

Polarized laser light scattering applied to surface morphology characterization of epitaxial III–V semiconductor layers

11 pages.-- PACS: 68.35.Bs; 81.05.Ea; 81.15.Hi; 78.35.+c; 78.66.FdIn this paper, we analyze typical morphologies of epitaxial III–V semiconductor layers by using a polarized laser light scattering technique. Crosshatched topographies, which are developed during heteroepitaxial growth, are studied. A sample with an intentionally high density of oval defects is also explored to establish how the laser light scattering pattern is affected by the presence of these defects, which are unavoidable in the epitaxial layers grown by molecular beam epitaxy. The former topographies produce a scattered light pattern that is highly anisotropic, with the intensity concentrated along two preferential directions; the latter defects give rise to a fairly isotropic pattern. Employing a perturbation-theoretical model, whose applicability and consistency are explicitly demonstrated by our results, the surface power spectral density is retrieved from the angle-resolved light scattering experimental data. For the samples exhibiting crosshatched topography, the scattering measurements provide information that allows us to model the roughness of the surface in terms of two quasi-one-dimensional, anisotropic components, and one two-dimensional, isotropic, long-range background. The root mean square heights and the typical lateral distances between ridges are obtained in quantitative agreement with the values extracted from the atomic force microscopy measurements. For the sample presenting oval defects, we consider their contribution to the surface power spectral density by means of a simple model of randomly distributed particles on a surface, and we compare the resulting power spectral density with typical behavior found in the literature for good-morphology GaAs layers. With the help of the ex situ information thus obtained, we also discuss the implementation of the light scattering technique for in situ monitoring during epitaxial growth.The authors wish to acknowledge the Spanish CICYT for financial support under Project No. TIC96-1020-C02. M.U.G. thanks the Consejería de Educación y Cultura de la Comunidad de Madrid for financial support. J.A.S.-G. acknowledges financial support from the Spanish DGESIC Grant No. PB97-1221.Spanish CICYTConsejería de Educación y Cultura de la Comunidad de MadridPeer reviewe

Rearrangement of the near-field landscape in heterogeneous nanoparticle arrays

Comunicación presentada en la Conferencia Española de Nanofotónica (CEN2012), celebrada en Carmona (Sevilla) del 1 al 4 de octubre de 2012.Geometric resonances in nanostructure arrays sustaining localized surface plasmons have received increasing attention in the last few years. These geometric resonances exhibit narrower peaks than the purely plasmonic ones, therefore these systems are promising candidates for sensing applications. Moreover, they provide new features in the near-field distribution patterns. Heterogeneous nanoparticle arrays offer a way to tune both the near and far-field response by using nanoparticles of different sizes, shapes or materials, but there is a lack of experimental studies in this context.Peer Reviewe

Magneto-optical effects in interacting localized and propagating surface plasmon modes

We report that the effect of an external magnetic field on the propagation of surface plasmons can be effectively modified through the coupling between localized (LSP) and propagating (SPP) surface plasmons. When these plasmon modes do not interact, the main effect of the magnetic field is a modification of the wavevector of the SPP mode, leaving the LSP virtually unaffected. Once both modes start to interact, there is a strong variation of the magnetic field induced modification of the SPP dispersion curve and, simultaneously, the LSP mode becomes sensitive to the magnetic field.This work was supported by the EU (NMP3-SL- 2008-214107-Nanomagma), the Spanish MICINN (“MAGPLAS” MAT2008-06765-C02-01/NAN and “FUNCOAT” CONSOLIDER INGENIO 2010 CSD2008-00023), the Comunidad de Madrid (“NANOBIOMAGNET” S2009/MAT-1726 and “MICROSERES-CM” S2009/TIC-1476), and CSIC (“CRIMAFOT” PIF08-016-4). We thank A. Cebollada and J. M. García-Martín for growing and characterizing the Au/Co/Au trilayers and reading this manuscript, and R. Quidant and G.Badenes for fruitful discussions.Peer reviewe

A growth method to obtain flat and relaxed In0.2Ga0.8As on GaAs (0 0 1) developed through in situ monitoring of surface topography and stress evolution

13 páginas, 3 figuras.-- PACS: 81.15.Hi, 81.05.Ea, 78.35.+c, 68.35.Bs.-- Comunicación oral presentada en el XI Molecular Beam Epitaxy (MBE-XI), Pekín (11/09/2000).In this paper we develop a growth process for obtaining flat and relaxed In0.2Ga0.8As layers on GaAs (0 0 1). The process designed is based on the results obtained by in situ and real time characterization of surface morphology and layer relaxation. In particular, our results show that for growth temperatures Ts200°C the relaxation of In0.2Ga0.8As layers is inhibited and the morphology does not evolve to a crosshatched pattern. After growth thermal treatments of these low-temperature (LT) In0.2Ga0.8As layers induce the development of a very faint (rms=0.5 nm) crosshatched-like morphology. The relaxation process during the thermal annealing is strongly asymmetric and the layers present a high final strain state. By growing on top of the LT layer another In0.2Ga0.8As layer at higher temperature, relaxation is increased up to R≈70% and becomes symmetric. Depending on the growth process of the top layers morphology evolution differs, resulting in better morphologies for top layers grown by atomic layer molecular beam epitaxy (ALMBE) at Ts=400°C. We have obtained 400 nm In0.2Ga0.8As layers with a final degree of relaxation R=70% and very flat surfaces (rms=0.9 nm).The authors wish to acknowledge the Spanish “CICYT” for financial support under Project No. TIC99-1035-C02. M.U. González and M. Calleja thank the Consejería de Educación y Cultura de la Comunidad de Madrid for financial support.Peer reviewe

High magneto-optical activity and low optical losses in metal-dielectric Au/Co/Au-SiO2 magnetoplasmonic nanodisks

Póster presentado en la Conferencia Española de Nanofotónica (CEN2012), celebrada en Carmona (Sevilla) del 1 al 4 de octubre de 2012.Magnetoplasmonics deals with the study of systems where the plasmonic and magneto-optical (MO) properties coexist and show a distinct interaction between them. In this kind of systems, the plasmonic properties can become tunable upon application of a magnetic field, or the MO effects can be largely increased by plasmon resonance excitation, as a consequence of the enhancement of the electromagnetic (EM) field in the MO component of the structure. The study of the enhanced MO activity in structures with subwavelength dimensions is especially interesting since they may be viewed as nanoantennas in the visible range with MO functionalities. The light harvesting properties of these systems upon plasmon resonance excitation brings as a consequence an enhanced EM field in its interior, and more interestingly in the region where the MO active component is present. In fact, it has been recently demonstrated that the MO enhancement can be directly linked with the amount of EM field inside the MO layer, in such a way that this layer can be used as a probe to determine the EM field distribution inside a nanostructure.Therefore, optimizing the EM field distribution within the structure by maximizing it in the MO regions while simultaneously minimizing it in all the other, non MO active, lossy components, will allow for the development of novel systems with larger MO activity and reduced optical absorption, becoming an alternative to state of the art dielectric MO materials, like garnets. Here we will present our approach to face this problem, based on the insertion of a dielectric layer in Au/Co/Au magnetoplasmonic nanodisks. The resulting nanostructure consists of two metallic nanodisks coupled through the dielectric layer, with one of the nanodisk being purely Au and the other one Au/Co (see Figure 1(a)). This kind of systems presents two hybridized localized plasmon resonance modes, showing two peaks in both the extinction and the MO spectra (see Figure 1(b)). Moreover, the EM field inside the nanostructure is strongly redistributed, being strongly concentrated in the nanodisk composed only of pure noble metal (Figure 1(c)). By optimizing the internal architecture (position of the Co layer), a configuration where the system exhibits large MO activity and low optical extinction in the same wavelength range can be obtained, as it can be seen in the right side of Figure 1(b) for the high wavelength peak.Peer Reviewe

Plasmonic and magnetoplasmonic nanostructures characterized by Scanning Near-field Optical Microscopy

Comunicación y presentación a Fuerzas y Tunel 2010.Most plasmonic devices are passive devices since their electromagnetic properties depend mainly on fixed properties such as the shape of the structures, their constitutive materials, and the dielectric media. A way to turn plasmonic devices into active ones is to use ferromagnetic metals, since due to their magneto-optical (MO) activity the optical response when applying an external low magnetic field can be modified. Unfortunately, plasmon resonances are critically broadened in ferromagnetic materials due to their important electromagnetic losses. An alternative is to combine ferromagnetic materials with noble metals. Recently, we have demonstrated that Au/Co/Au nanodisks exhibit enhanced magnetoplasmonic properties such as a significant increase of the MO activity when the localized surface plasmon (LSP) resonance is excited [1,2].Peer reviewe

Magnetoplasmonic Interferometers and Applications

Comunicación presentada en el 2nd Early Stage Researchers Workshop in Nanoscience, celebrado en Madrid el 28 y 29 de junio de 2012.Surface plasmons polaritons (SPP) are evanescent waves that propagate along a dielectric-metal interface. They can be confined in subwavelength metal structures, i.e. below the diffraction limit, which leads to many possible applications, including miniaturized optical devices. Within that context, the development of active plasmonics is important to achieve nanophotonic devices with advanced functionalities. This requires a system where the plasmon properties can be manipulated using an external agent. Among the different control agents considered so far, the magnetic field seems a promising candidate, since it is able to modify the dispersion relation of SPP at reasonable magnetic field strengths, and with a high switching speed. This modulation comes from the non-diagonal elements of the dielectric tensor, εij, appearing when the magnetic field is turned on. For noble metals, the ones typically used in plasmonics, these elements are proportional to the applied magnetic field but, unfortunately, very small at field values reasonable for developing applications. On the other hand, ferromagnetic metals have sizeable εij values at small magnetic fields (proportional to their magnetization), but are optically too absorbent. A smart system to develop magnetic field tunable plasmonic devices is the use of multilayers of noble and ferromagnetic metals. That is the framework of the present work, where we analyze the magnetic field induced SPP wavevector modulation (Ak) in Au/Co/Au films as a function of the wavelength and its possible application as a sensor.N

Localized surface plasmon resonance effects on the magneto-optical activity of continuous Au/Co/Au trilayers

We study how the magneto-optical activity in polar configuration of continuous Au/Co/Au trilayers is affected by the excitation of localized plasmon resonances of an array of Au nanodiscs fabricated on top of them over a dielectric SiO2 spacer. We show that the effect of the nanodiscs array is twofold. First, it optimizes the absorption of light at specific photon energies corresponding to the localized surface plasmon excitation of the array, modifying the reflectivity of the system (we define this effect as the purely optical contribution). Second, upon localized plasmon resonance excitation, the electromagnetic field in the whole system is redistributed, and an enhanced magneto-optical activity occurs at those energies where the electromagnetic field in the magnetic layer is increased (this effect is identified as the purely magneto-optical contribution of the nanodiscs array).This research was carried out with the financial support of the Spanish Ministry of Science and Education (NAN2004-09195-C04 and MAT2005-05524-C02-01), Comunidad de Madrid (S-0505/MAT/0194 NANOMAGNET) and the European Commission through the NoE PHOREMOST (FP6/2003/IST/2-511616). M. U. G. thanks the Spanish Ministry of Education for funding through the “Ramón y Cajal” program.Peer reviewe